1. Field of the Invention
This invention relates generally to an improved engine retarding system of the compression release type. More particularly, the invention relates to a turbocharged internal combustion engine equipped with a compression release retarder wherein the turbocharger comprises a double entry turbine together with an improved diverter valve and the method of operating such apparatus to provide improved engine retarding and improved engine performance.
2. The Prior Art
The problem of providing adequate and reliable braking or retarding for vehicles, particularly large tractor-trailer vehicles is well known. When such vehicles are operating at normal highway speeds they possess a very large momentum, and this may be increased substantially when the vehicle begins to accelerate down a long decline. While the normal drum or disc type wheel brakes are capable of absorbing a large amount of energy over a short period of time, the absorbed energy is transformed into heat which rapidly raises the temperature of the braking mechanism to a level which may render ineffective the braking surfaces and other parts of the mechanism. Repeated use of the wheel brakes under these conditions is impracticable and, therefore, resort has been made to auxiliary retarding devices.
Such auxiliary devices include hydraulic or electrodynamic retarding systems wherein the kinetic energy of the vehicle is transformed by fluid friction or magnetic eddy currents into heat which may be dissipated through appropriate heat exchangers. Other auxiliary retarding systems include exhaust brakes which inhibit the flow of exhaust gases through the exhaust system and compression release mechanisms wherein the energy required to compress the intake air during the compression stroke of the engine is dissipated by exhausting the compressed air through the exhaust system. With respect to the exhaust brake, a portion of the kinetic energy of the vehicle is dissipated through the engine cooling system. With the engine compression release retarder a portion of kinetic energy of the vehicle is dissipated as heat through the engine cooling system while another portion of the kinetic energy is dissipated in the form of hot air through the engine exhaust system.
One principal advantage of the engine compression release retarder and the exhaust brake over hydraulic and electrodynamic retarders is that both of the latter retarders require dynamos or turbine equipment which may be bulky and expensive in comparison with the mechanism required for the usual exhaust brake or engine compression release retarder. A typical engine compression release retarder is shown in the Cummins U.S. Pat. No. 3,220,392 while a typical exhaust brake is disclosed in Benson U.S. Pat. No. 4,054,156.
The concept of supercharging an engine in order to increase the mass flow of air has long been established in the art for both land vehicles and aircraft. Such supercharging is frequently accomplished by means of a turbocharger comprising an exhaust gas turbine coupled with a centrifugal compressor. While various types of turbines may be employed, a vaneless radial in-flow turbine is most commonly used for automotive turbochargers. In such a turbine it may be desirable to provide a divided volute for considerations of performance and fuel economy. In such a divided volute, it would then be possible to direct the exhaust gas flow to one or both scrolls of the volute as desired. Such a construction is shown in Webster U.S. Pat. No. 3,557,549, Neff U.S. Pat. No. 3,614,259, Clarke U.S. Pat. No. 2,247,151, Cowland U.S. Pat. No. 2,838,907, Connor U.S. Pat. No. 3,270,495, and Nancarrow U.S. Pat. No. 3,423,926. Where a divided volute turbine is used, it is necessary to provide means to direct the gas flow to one or both scrolls of the volute. The patents cited immediately above disclose various forms of diverter mechanisms for this purpose. Additional diverter valves are shown in Woollenweber, Jr. U.S. Pat. No. 4,008,572, Navarro U.S. Pat. No. 3,559,397, Kofink U.S. Pat. No. 3,137,477, Nancarrow U.S. Pat. No. 3,313,518, and Morgulis U.S. Pat. No. 3,975,911.
As disclosed in various of the patents cited above, the volume of exhaust gas available to drive the turbine of the turbocharger varies with the engine speed and engine throttle settings. A decrease in exhaust gas volume results in a decrease in the gas velocity at the turbine nozzles and a reduced turbine efficiency as well as reduced speed. It has been found desirable, in order to improve the efficiency of the turbine at low exhaust gas flow rates, to divert all of the exhaust gas to one scroll of the turbine or to selected inlet nozzles so as to maintain the gas velocity at as high a level as possible at the entry to the turbine wheel. As shown in the Webster U.S. Pat. No. 3,557,549 and the Neff U.S. Pat. No. 3,614,259, for example, a diverter valve of the flapper type directs exhaust gas into one or both scrolls of the divided volute of the turbine. In the Clarke U.S. Pat. No. 2,247,151 sequentially actuated diverter valves direct the exhaust gas into various segments of the turbine. Another approach is represented by the Kofink U.S. Pat. No. 3,137,477 which discloses the concept of changing the number of nozzles through which the exhaust gas flows.
The concept of employing a turbocharged engine equipped with a double entry turbine and a compression release engine retarder in combination with a diverter valve to direct the flow of air through one scroll of the divided volute of the turbine during an engine retarding mode of operation is disclosed in U.S. application Ser. No. 407,642 filed Aug. 12, 1982 (now U.S. Pat. No. 4,395,884) which is a continuation of application Ser. No. 238,588 which, in turn, is a continuation of application Ser. No. 21,445 filed Mar. 19, 1979 and assigned to the assignee of the present application. See also German publication O.S. 2,820,941 published Nov. 23, 1978 which discloses a combination of a turbocharged engine fitted with a compression release engine retarder. According to the German patent application the turbine is provided with a means to change the speed of the incoming gas fed to the turbine wheel.